In contrast with conventional differential assemblies, the worm-type does not utilize a bevel gear arrangement. Rather, it includes worm (or helical) gears, coupled to each inner axle end as drive gears, together with so called balancing or transfer gears associated with each of the worm gears and in mesh with each other for transferring and dividing torque between the axle ends. The transfer gears, or combination gears as they are referred to herein, are mounted in pairs, and each combination gear of a pair rotates on an axis of rotation that is substantially parallel to a tangent of the envelope of an associated axle drive, or side gear. Each combination gear consists of a centrally located hourglass worm wheel portion flanked at either end by an integral spur gear portion.
This invention has particular applicability to the hobbing processes used to rough form or finish form hourglass worm wheels or hourglass worm wheel portions of combination gears such as are used in worm-type differential assemblies.
In the gear making art, it is a generally accepted practice to provide some degree of mismatch between mating gears in order to accommodate a range of manufacturing tolerances, mounting errors and tooth deflections under load. Tip relief, for example, is an arbitrary modification of tooth profile whereby a small amount of material is removed near the tip of a gear tooth. Crowning, where material is removed from a gear tooth in the lengthwise direction, is another example of conventional gear modifications. To provide tip relief or crowning, the finishing hob is given a slight modification in form.
The present invention concerns a novel technique for removing material from a specific type of gear, i.e., an hourglass worm wheel, to provide a predetermined degree of mismatch between the hourglass worm wheel and a mating helical gear for the purpose of improving overall gear performance. The term "hourglass" as used in connection with the present invention, refers generally to the trace of tooth root surfaces in a worm wheel blank, the outer diameter shape of which may be of the form of an hourglass or a cylinder.
Hourglass worm wheels are conventionally produced by what is known as the radial infeed, or plunge, cutting method. In this method, the rotating hob, which duplicates the tooth form of the helical gear that ultimately is to mesh with the worm wheel (except that the tooth height may be increased for clearance and the tooth thickness may be varied for backlash) is fed in a radial direction toward the axis of the rotating hourglass blank. When the proper depth of cut has been reached, the hob is withdrawn in a radially outward direction. This same procedure is used for both rough forming and finish forming operations, using the appropriate class of hob.
By way of additional background, helical gears are typically formed by passing the rotating hobbing tool axially along the face of the rotating blank in a direction parallel to the longitudinal axis of the blank.
In the past, the desired lengthwise mismatch between mating worm and worm wheel teeth was accomplished through the utilization of an oversize hob, selected in accordance with known formulas. It has been found, however, that when oversize hobs are used, i.e., a hob larger than the helical gear it represents, backlash (the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the operating pitch circle) is eliminated or rapidly taken up upon slight axial shifting of the worm wheel. Absent sufficient backlash under load, noise generating edge contacts and/or interferences are experienced, leading to premature surface deterioration.
In addition, it is also known that when oversize hobs are used to obtain a desired amount of lengthwise mismatch, subsequent sharpenings of the hob may either increase or decrease the amount of mismatch provided by the hob.
The present invention seeks not only to eliminate such problems which accompany prior art hobbing techniques, but also to improve the overall performance characteristics of worm-type differential assemblies.